A Comparison of Fe4S4Clusters in High-Potential Iron Protein and in Ferredoxin

The structures of both oxidized (HPox) and reduced (HPred) high-potential iron protein and of oxidized ferredoxin (Fdox) have been partially refined at 2.0-Å resolution by methods similar to those applied to the protein rubredoxin [Watenpaugh, K. D., Sieker, L. C., Herriott, J. R. & Jensen, L. H. (1971) Cold Spring Harbor Symp. Quant. Biol.36, 359-367]. Average bond lengths and angles in the HPredand FdoxFe4S4*clusters are the same to within the root-mean square (rms) deviation of each mean value. A preliminary comparison of the two HiPIP oxidation states indicates that the HPoxcluster is geometrically similar to the HPredcluster, but that it is smaller by 0.1-0.2 Å in certain dimensions. The HiPIP and ferredoxin cluster geometry is also nearly identical to that reported recently for a synthetic analog [Herskovitz, T., Averill, B. A., Holm, R. A., Ibers, J. A., Phillips, W. D. & Weither, J. F. (1972) Proc. Nat. Acad. Sci.USA 69, 2437-2441]. An apparent paradox presented by the large difference between the HiPIP and ferredoxin electrode potentials can be resolved by the assumption that the Fe4S4*cluster has not two but threeoxidation states. The fully oxidized (HPox) and fully reduced (Fdred) clusters are paramagnetic, and the intermediate state is spin-paired [Tsibris, J. C. M. and Woody, R. W. (1970) Coord. Chem. Rev.5, 417-458]. This hypothesis is supported by structural and spectroscopic evidence that the “paired-spin” state exists in both HPredand Fdox.